『Abstract
Despite the high priority of wetland restoration as the primary
measure to reduce agricultural nutrient loads, it is also widely
recognized that wetlands restored on former agricultural land
could potentially release accumulated phosphorus (P) and become
a source of eutrophication. Simulating the upward percolation
of groundwater with variable O2 content and
flow rate, we investigated the hydro-biogeochemical Fe and P dynamics
in intact cores of a carbon rich peat and carbon poor sand. Percolation
of deionized water with high, low or no O2
supply at 10℃ caused markedly different in situ redox regimes
in the two soils during 21 or 67 days of continuous percolation
at either 1 or 4 mm h-1. Anoxic conditions occurred
in the peat soil at both low oxygen supply and anoxic infiltration,
causing reductive Fe(III) dissolution with high Fe(II) and P effluent
concentrations and total P (TP) release rates from 7.6 to 11 mg
P m-2 day-1. Organic or particulate P contributed
to 40-45% of total P losses from the peat. In contrast, the high
O2 supply during high flow rate kept the
peat oxic and lowered TP release rates to 6.7 mg P m-2
day-1. The carbon poor sand demonstrated that this
soil type is not exposed to reducing conditions, unless the soil
is actually infiltrated by anoxic water, causing an increase in
TP release rates from 5.8 (oxic) to 7.6 (anoxic) mg P m-2
day-1. Hydrology turned out as a major factor controlling
TP release, as a four-fold increase in flow rate increased the
P release rate in the sand from 5.8 to 24 mg P m-2
day-1. Thus, oxic conditions caused the sand to be
the most critical soil in terms of P release with diffusion-limited
desorption as the major mechanism, while at anoxic conditions
P release following reductive Fe(III) dissolution in the peat
soil was quantitatively most important. Effluent Fe(II): PO4-P molar ratios ranged between 10 and 40 (peat)
and <3 (sand). Phosphorus release rates and soil P fractions
indicate that both of these soils can potentially release P for
several decade, however, reoxidation and readsorption of P at
the redox boundary should be considered in terms of actual environmental
impact.
Keywords: Wetland restoration; Redox chemistry; Hydrology; Phosphorus;
Desorption; Reductive iron dissolution』
1. Introduction
2. Materials and methods
2.1. Field sites and soil characteristics
2.2. Experimental setup
2.3. Experimental design and convective flow experiment
2.4. Effluent analysis
3. Results
3.1. Geochemical and physical soil characteristics
3.2. Dynamics in redox conditions, electric conductivity and
pH
3.3. Iron and phosphorus mobilization
3.4. Phosphorus and iron forms
4. Discussion
4.1. Active flow volume and redox regimes
4.2. Phosphorus and iron mobilization in sand
4.3. Phosphorus and iron mobilization in peat soils
4.4. Column versus batch experiments
5. Conclusion and environmental implications
Acknowledgments
References